SOX2 and craniosynostosis: We found that the Twist1flox/+; Sox2-Cre mice developed craniosynostosis - a phenotype that is similar to the Twist1 heterozygous mice [11], [32], characterized by much narrower sagittal and interfrontal sutures and had additional toe originating from a duplicated metatarsal as demonstrated by alcian blue and alizarin red staining (Figure S3A–D).